1. Field of the Invention
The present invention relates to a rotary connector incorporated in an automotive steering unit and used, for example, as an electrical connection between an air bag device mounted on a steering wheel of an automobile and associated control circuitry mounted on a body of the automobile. More particularly, the present invention relates to a rotary connector in which flat cables are stored in a holding space formed between a fixed housing and a movable housing, wherein the flat cables are wound in opposing directions via U-shaped reversal portions.
2. Description of Related Art
A rotary connector includes a fixed housing, a movable housing which is rotatably connected to the fixed housing, and a flat cable stored in a holding space formed by the fixed housing and movable housing. The flat cable has a first end connected to a joint (connector) mounted on the fixed housing, and a second end connected to a joint mounted on the movable housing. Rotary connectors are incorporated in an automotive steering unit and employed, for example, to provide an electrical connection between an air bag inflator attached to a steering wheel of an automobile, which can be turned a finite number of revolutions, and associated control circuitry mounted on a body of the automobile. The flat cable is shaped like a band and supports a plurality of parallel conductors which extend between the first and second ends.
Two types of rotary connectors are commonly used: in the first type, the flat cable is wound in single direction (i.e., clockwise or counterclockwise) to form a coil, and in the second type, the flat cable is partially wound in a first direction, then reversed via a U-shaped reversal portion and partially wound in the opposite direction. In the second (reversal) type rotary connector, the required length of the flat cable is significantly reduced relative to that needed for the first (coiled) type rotary connector.
Reversal type rotary connectors typically use one flat cable with the plurality of conductors arranged in parallel. As the number of conductors is increased to meet a recent trend toward multiple-circuit applications, a problem arises in that the width of the flat cable must inevitably increase to support more conductors, thereby increasing the size of the rotary connector.
One solution to this problem is incorporated into the rotary connector disclosed in U.S. Pat. No. 3,763,455, which supports multiple-circuit applications by employing two flat cables to accommodate the conductors.
FIG. 4 is a top plan view illustrative of a schematic configuration of a rotary connector disclosed in U.S. Pat. No. 3,763,455. A movable housing 101, which has a cylindrical inner section, is rotatably connected to a fixed housing 100 having a cylindrical outer section. A first flat cable 103 and a second flat cable 104 are housed in an annular holding space 102 formed between the fixed housing 100 and the movable housing 101. These flat cables 103 and 104 are stored in the holding space 102 such that they are wound in a first (counterclockwise) direction around the outer cylindrical section of the fixed housing 100, and in a second (clockwise) direction around the inner cylindrical section of the movable housing 101. At the position where the winding direction is reversed, U-shaped reversal portions 103a and 104a are formed. The inner ends of the two flat cables 103 and 104 are connected to cable outlet sections 107 and 108 disposed adjacent to the inner cylindrical section of the movable housing 101. The outer ends of the two flat cables 103 and 104 are connected to cable outlet sections 109 and 110 located adjacent the outer cylindrical section of the fixed housing 100. Further, disposed in the holding space 102 are first and second groups of rollers 105 and 106 arranged in the circumferential direction, the reversal portion 103a of the first flat cable 103 being bent around the first group of rollers 105, and the reversal portion 104a of the second flat cable 104 being bent around the second group of rollers 106.
In the rotary connector thus configured, when, for example, the movable housing 101 is turned clockwise (relative to the fixed housing 100 in FIG. 4), the reversal portions 103a and 104a of the flat cables 103 and 104, respectively, move clockwise in the holding space 102 by an amount of which is smaller than that of the movable housing 101, causing the flat cables 103 and 104 to be wound onto the inner cylindrical section of the movable housing 101. Conversely, when the movable housing 101 is turned counterclockwise in FIG. 4, the reversal portions 103a and 104a of the flat cables 103 and 104, respectively, also move counterclockwise by an amount which is smaller than that of the movable housing 101, causing the flat cables 103 and 104 to be unwound from the outer cylindrical section of the fixed housing 100. At the time of such winding and unwinding, the groups of rollers 105 and 106 also move in the same direction in response to forces applied by the reversal portions 103a and 104a of the flat cables 103 and 104, respectively.
In the conventional rotary connector described above, the radial deformation of the two flat cables 103 and 104 is restricted by the groups of rollers 105 and 106. Hence, the flat cables 103 and 104 can be smoothly moved in the circumferential direction of the holding space 102. However, because the groups of rollers 105 and 106 are disposed in the holding space 102 so that they are separated from each other, vibrations applied to the rotary connector cause the groups of rollers 105 and 106 to bump against each other, producing a collision noise. Another problem associated with this rotary connector is that it is difficult to assemble the groups of rollers 105 and 106 onto the rotary connector.
One alternative arrangement addressing the problem described above is shown in FIG. 5. In this structure, instead of employing the mutually separated groups of rollers 105 and 106, a moving member 111 is movably disposed in the holding space 102 which has at least two openings 111a and 111b, and the reversal portions 103a and 104a of the flat cables 103 and 104 are passed through the openings 111a and 111b. The moving member 111 is composed of a ring-like rotary plate 112 and a plurality of rollers 113 rotatably connected to the rotary plate 112. The openings 111a and 111 are formed at equal intervals between the groups of the rollers 113. Hence, when the moving member 111 which has such openings 111a and 111b is employed, the rotary plate 112 and the rollers 113 can be mounted into the rotary connector during assembly as one piece, and bumping between the rollers is prevented, thus solving the problems mentioned above with respect to the conventional rotary connector. However, the rotary connector shown in FIG. 5 poses different problems which are described below.
Referring again to FIG. 5, the diameter of the inner cylindrical section of the movable housing 101 is denoted as D, and the thickness of each of the flat cables 103 and 104 is denoted as t. With this arrangement the winding diameter of the first flat cable 103 directly wound on the inner cylindrical section is D, whereas the winding diameter of the second flat cable 104 wound on the inner cylindrical section via the first flat cable 103 is (D+2t). Therefore, the winding diameters of the two flat cables 103 and 104 do not coincide. Accordingly, when the movable housing 101 is turned, the amount of winding of the flat cable 103 onto the inner cylindrical section is different from that of the flat cable 104, or the amount of unwinding of the two flat cables therefrom is different, thereby causing the reversal portion 104a of the second flat cable 104 which has the larger winding diameter to move faster than the reversal portion 103a of the first flat cable 103 which has the smaller winding diameter. As a result, as shown in FIG. 5, when the flat cables 103 and 104 are unwound from the inner cylindrical section and wound back onto the outer cylindrical section, the reversal portion 104a of the second flat cable 104 comes in contact with the roller 113 located at the end of the opening 111b, whereas the reversal portion 103a of the first flat cable 103, which is moving slower, merely moves in the opening 111a and does not come in contact with the roller 113, thus presenting a problem in that the moving member 111 cannot be smoothly moved by the forces applied by the two reversal portions 103a and 104a.